The present invention relates in general to a biological treatment of wastewater, e.g. sewage.
The treatment of wastewater, whether for purposes of recycling or prior to its discharge into treatment works, rivers, lakes, groundwater suppliers, etc., is an ever-increasing problem. To date, three general classes of methods for removing contaminating organic substances from wastewater, such as sewage, have been developed. These are chemical treatments, biological treatments, and physical treatments.
Chemical treatment generally involves the introduction of a strong oxidizing agent, such as ozone, chloride dioxide and hydrogen peroxide, into the wastewater stream, whereby the contaminating organic substances within the wastewater are partially or fully oxidized to carbon dioxide and water. Whilst in some instances, such chemical purification may be desirable, it often may pose problems. First, hazardous chemical agents are required during the treatment. Secondly, such agents may be relatively expensive. Thirdly, on a large scale, oxidation processes may be rather slow, unless the oxidant is used in very large amounts.
Physical treatment includes, for example, carbon adsorption and air stripping. Though such methods are effective for some applications, they are non-destructive of toxic organic contaminants. Thus, disposal problems of such toxic organic contaminants remain.
Biological treatments have been used in a wide variety of applications. Generally, the treatment involves contacting wastewater with a consortium (community) of microorganisms that utilize dissolved organic substances as nutrients. During the biological treatment, three main activities occur: reduction of biological oxygen demand (B.O.D reduction), nitrification and denitrification of the organic waste. All three processes are affected by bacteria, the former twoxe2x80x94by aerobic bacteria, and the latterxe2x80x94by anaerobic (anoxic) bacteria.
In the various reactors for biological treatment of sewage, mutual disposition of the biological activities in the overall treatment may be different in that the denitrification stage may be performed before, concurrently or after B.O.D. reduction.
When denitrification is performed before B.O.D. reduction and nitrification, this may take place either in a separate reactor or in the area of the main reactor where the is raw sewage enters. When denitrification is performed after B.O.D. reduction and nitrification, the system typically requires its supplementation with an additional source of carbon, such as methanol, in order to effect denitrification. When denitrification, B.O.D. reduction and nitrification occur concurrently, in a so called combined system, this system typically comprises alternating aerobic and anaerobic stages in which incremental reduction in the organic carbon and nitrogen content of the sewage is accomplished in each stage. This enables the system to maintain the organic carbon after the B.O.D. reduction stages at a sufficient level for denitrification without adding an additional source of carbon. Systems of this type are disclosed, for example, in U.S. Pat. Nos. 3,994,802; 3,945,918; 4,279,753; 4,564,457 and 4,374,730, in Barnard J. L. Water Research 9:485-490 (1975) and in Drews J. L. C. et al. Water Research 7:1183-1194 (1973).
Typically, the combined systems for biological treatment of sewage hitherto known are designed to include the use of aeration and/or agitation means during the aerobic stage of the treatment for the purpose of reduction of the time required for nitrification. However, the use of such means, in fact, results in the non-uniform distribution of the dissolved oxygen in the treated sewage, which leads to incomplete sewage treatment. Furthermore, simple calculations made by the applicant have shown that, by employing such means, the amount of air per cubic meter of sewage that is supplied thereto is 70-80% more than necessary for the performance of the aerobic activities.
The object of the present invention is to provide a novel combined system for biological treatment of sewage.
In accordance with one aspect of the present invention there is provided a biological reactor for the treatment of sewage, that is void of any elector-mechanical aeration or agitation means and comprises:
at least one array of successively alternating aerobic and anaerobic compartments having liquid passage therebetween, for, respectively, aerobic and anaerobic degradation of the biodegradable substances in said sewage, each compartment having a bottom, side walls and an open top area defined between said side walls;
each pair of successive aerobic and anaerobic compartments having a common partition wall via which said liquid passage is provided, wherein the liquid passage from an aerobic compartment to its adjacent succeeding anaerobic compartment is provided through a slot at the bottom of their common wall, and the liquid passage from an anaerobic compartment to its adjacent succeeding aerobic compartment is provided by means of gravity-caused liquid flow over their common wall.
According to another aspect of the invention, there is provided a process for biologically treating sewage, wherein said sewage is transferred, by gravitational forces with a substantially constant flow rate (Q), through an array of successively alternating aerobic and anaerobic compartments for, respectively, aerobic and anaerobic degradation of biodegradable substances in said sewage, the compartments being separated by partition walls and each having a top area (S), the passage of said sewage from an aerobic compartment to its adjacent succeeding anaerobic compartment being provided at the bottom of their common partition wall, while the transfer of sewage from an anaerobic compartment to its adjacent aerobic compartment is performed by means of gravity-caused flow over their common partition wall; whereby oxygen required for said aerobic treatment is dissolved into the sewage via the air-sewage interface at the top area of the anaerobic and aerobic compartments forming thereby a dissolution layer, the depth of oxygen dissolution and, consequently, the oxygen concentration in the dissolution layer at the top area of the compartments being defined by the vertical velocity of the liquid within the compartments, calculated as V=Q/S.
Within the context of the present invention, an xe2x80x9caerobic compartmentxe2x80x9d is a compartment wherein the aerobic activities substantially take place, whereas, an xe2x80x9canaerobic compartmentxe2x80x9d is a compartment wherein the anaerobic activity substantially take place.
The term xe2x80x9cdissolution layerxe2x80x9d refers, in general, to a surface layer of the liquid characterized by the depth of liquid at which oxygen is dissolved. The depth of the layer may vary, depending on the flow rate of the liquid in the reactor, so that the slower the flow rate, the tinner the dissolution layer and vice versa. Oxygen is dissolves into the liquid via the top area of the aerobic and anaerobic compartments. Thus, the top surface of the whole reactor serves as an air-liquid contact area, from which oxygen is dissolved into the sewage, at a constant oxygen concentration. By utilizing the air-liquid contact area as the only source of oxygen required to perform the treatment, the need for mechanical aerators or air compressors is obviated, which is economically beneficial.
It should be noted that though the reactor of the invention is designed to perform only the biological treatment of sewage it may easily form part of a sewage treatment array, being connected to other sewage treatment facilities, known to the man of the art.